Switch to new pytest runner

[mandli]

This PR changes the way we are doing testing on a larger scale and adds some new capabilities:

• Uses pytest more directly than the previous tests based on UnitTest
• Significant code simplification of both the test scaffoloding and individual tests
• Gives full control of the output (temporary) directory to the test via the commandline pytest --basetemp=
• Can specify which setrun is used by path and removes problems with collision of names
• Uses some pytest fixtures to provide other functionality meaning that they can also be expanded

There are a number of other advantages to this approach including using other plugins for PyTest in the future.

[mandli]

We can merge this or not, but it provides a proof of concept that we could do this. I think this may help a lot with more complex testing, such as with the adjoint tests, and help with the use of alternative setrun.py files from arbitrary locations.

[mandli]

Moved the generic code for the test runner into clawutil now, see clawpack/clawutil#96 for details.